Open a popup with a POST request

In my application I have an import from XML feature. The XML data structure is complex (using XML Records) and is used for application backoffice configuration. When the user loads the xml file, in a popup is shown all the file structure. Some of the fields are binary files in base64. The import feature is working fine. I show the file list in an table records. In each table records line I need to show a preview of each file in a popup before the files are saved into the application. For small files all is working great, but there are files with a length way too big for a GET request (>10k char)...